Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a core barrel assembly is used to cut a core from the subterranean formation and to transport the core to the surface for analysis. Analysis of the core can reveal invaluable data concerning subsurface geological formations—including parameters such as permeability, porosity, and fluid saturation—that are useful in the exploration for and production of petroleum, natural gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.
A conventional core barrel assembly typically includes an outer barrel having, at a bottom end, a core bit adapted to cut the core and to receive the core in a central opening, or throat. The opposing end of the outer barrel is attached to the end of a drill string, which conventionally comprises a plurality of tubular sections, that extends to the surface. An inner barrel assembly having an inner tube configured for retaining the core is located within and releasably attached to the outer barrel. The inner barrel assembly further includes a core shoe disposed at one end of the inner tube adjacent the throat of the core bit. The core shoe is configured to receive the core as it enters the throat and to guide the core into the inner tube. Both the inner tube and core shoe are suspended within the outer barrel with structure permitting the core bit and outer barrel to rotate freely with respect to the inner tube and core shoe, which may remain substantially rotationally stationary or which may rotate limitedly due to frictional forces. Thus, as the core is cut—by application of weight to the core bit through the outer barrel and drill string in conjunction with rotation of these components—the core will traverse the throat of the core bit to eventually reach the substantially rotationally stationary core shoe, which accepts the core and guides it into the inner tube assembly where the core is retained until transported to the surface for examination.
Conventional core bits are generally comprised of a bit body having a face surface on a bottom end. The opposing end of the core bit is configured, as by threads, for connection to the outer barrel. Located at the center of the face surface is the throat, which extends into a hollow cylindrical cavity formed in the bit body. The face surface includes a plurality of cutters arranged in a selected pattern. The pattern of cutters includes at least one outside gage cutter disposed near the periphery of the face surface that determines the diameter of the bore hole drilled in the formation. The pattern of cutters also includes at least one inside gage cutter disposed near the throat that determines the outside diameter of the core being cut.
During coring operations, a drilling fluid is usually circulated through the core barrel assembly to lubricate and cool the plurality of cutters disposed on the face surface of the core bit and to remove formation cuttings from the bit face surface to be transported upwardly to the surface through the annulus defined between the drill string and the wall of the well bore. A typical drilling fluid, also termed drilling “mud,” may be a hydrocarbon or water base in which fine-grained mineral matter is suspended. The core bit includes one or more ports or nozzles positioned to deliver drilling fluid to the face surface. Generally, a port includes a port outlet, or “face discharge outlet,” which may optionally comprise a nozzle, at the face surface in fluid communication with a face discharge channel. The face discharge channel extends through the bit body and terminates at a face discharge channel inlet. Each face discharge channel inlet is in fluid communication with an upper annular region formed between the bit body and the inner tube and core shoe. Drilling fluid received from the drill string under pressure is circulated into the upper annular region to the face discharge channel inlet of each face discharge channel to draw drilling fluid from the upper annular region. Drilling fluid then flows through each face discharge channel and discharges at its associated face discharge port to lubricate and cool the plurality of cutters on the face surface and to remove formation cuttings as noted above. Drilling fluid may also be circulated through the through of the coring bit or through other discharge channels, ports, and nozzles that may be provided at the core bit.
Also during the coring operations, debris, generally in the form of formation cuttings separate from the core, may enter the through of the coring bit and may be transported upwardly toward the core shoe. Accordingly, when the core is cut and traverses upwardly through the throat of the coring bit toward the core barrel assembly, the core may push debris between the core catcher and the core shoe. Consequently, the debris in combination with the upward motion of the core may cause a portion of the core catcher to deform such that the core catcher may pass into the inner barrel assembly in which it is intended to retain the core. Such deformation may result in failure of the core catcher and the coring operations.